The present invention relates to a hydraulic radial piston pump having a set of pistons arranged radially around the rotary shaft for sequential reciprocating motion for suction and delivery of working oil.
A prior-art, hydraulic radial piston pump is arranged, for example, as shown in FIG. 6, in which an eccentric cam 3 is rigidly mounted onto the rotary shaft 2 and installed within the inner oil chamber 4 formed in the pump casing 1. Each one of the radially arranged cylinders 1A provided in the pump casing 1 accommodates a piston 6 which can slide therein and is actuated by the eccentric cam 3. The inner oil chamber 4 communicates with a reservoir, not shown, and hence, is kept filled with working oil.
The head of the piston 6 extends inwardly into the inner oil chamber 4. The outward end of the cylinder 1A is closed with a cap 7 fitted into the pump casing 1. There is provided an outer oil chamber between the cap 7 and the back of the piston 6. The head of the piston 6 is pressed against the periphery of the eccentric cam 3 by a spring 8 interposed between the cap 7 and the back of the piston 6.
A suction orifice 21 is made at the head end side of the piston 6 for communication with the inner chamber 4. The suction orifice 21 opens when the piston 6 loaded by the spring 8 advances into the inner oil chamber 4 following the movement of the cam 3 and closes when the piston 6 retracts into the pump casing 1 through the outward stroke movement.
The piston 6 is thus displaced by the rotary motion of the cam 3 as shown in FIG. 7. More specifically, the piston 6 first slides from the maximum compression position (top dead center) inwardly so as to enlarge the outer oil chamber 5 and open the suction orifice 21 for introduction of working oil from the inner oil chamber 4 into the outer oil chamber 5 and secondly, after reversing its sliding motion at the maximum extended position (bottom dead center), retracts into the pump casing 1 with the suction orifice 21 being closed so as to pressurize the working oil in the outer oil chamber 5 and then, discharge it via a check valve through a delivery passage not shown. The piston 6 repeats this cycle at periodic intervals corresponding to the rotational angle of the eccentric cam 3. Accordingly, the working oil is sequentially discharged from the outer oil chambers 5 to their respective delivery passages.
The prior-art, pump, however, has such a disadvantage that the noise and vibration during operation increase and thus, the durability decreases. This results from the cavitation generated due to a drop in the internal pressure of the outer oil chamber 5 during the extension of the outer oil chamber 5 without introduction of working oil shown in FIG. 7, the pulsation of delivery pressure created due to a short delivery period defined between 240 to 350 degrees, and the abrupt change of pressure caused at the start of delivery due to a faster sliding speed of the piston 6 approximately at the 240 degrees stage. Also, the effective length of stroke of the piston 6 is short and it is thus necessary to increase the eccentricity of the cam 3. Furthermore, as shown in FIG. 8, the head of the piston 6 comes into linear contact with the cam 3 and creates a considerably high rate of surface contact pressure. This causes their contact portions to wear quickly and simultaneously, produce more noise during the operation.
Another radial piston pump using a plurality of pistons arranged around an eccentric cam is disclosed in Japanese Patent Provisional Publication No. 47-12684, in which during the inward motion of a piston actuated by the rotary motion of the eccentric cam, working oil is introduced into the oil chamber provided behind the piston through a groove formed in the eccentric cam and a flow port formed in the head of the piston. This radial piston pump allows the eccentric cam to rotate at a relatively high speed with respect to the piston head and thus, causes either of them to wear quickly. It is also acknowledged that the problem of incidental noise is not yet solved. Because the difference of relative operating speed is great between the groove and the flow port, it is difficult to determine the timing for opening and closing motions.
U.S. Pat. No. 3,003,462 discloses a radial piston pump in which the reciprocating motion of a plurality of pistons arranged around an eccentric cam is actuated by the eccentric motion of a polygonal ring fitted onto the periphery of the eccentric cam. This radial piston pump includes a couple of one-directional valves for suction and delivery respectively so that working oil can be sucked in and discharged by the extension and reduction of an oil chamber provided behind each piston. Accordingly, its arrangement becomes elaborate and its pump casing becomes large in size. Additionally, the suction is controlled by the one-directional suction valve and its timing can hardly be determined arbitrarily.
The purpose of the present invention is thus to eliminate the disadvantages of the foregoing prior-art, radial piston pumps.
It is the primary object of the present invention to provide a radial piston pump which has a simpler arrangement, improved durability, reduced mechanical noise, and can be adapted for arbitrary setting of the suction timing.